1. Field of the Invention
The present invention relates to a method and apparatus for generating feedback in a Cooperative Multi-Point (CoMP) system in which a plurality of Base Stations (BSs) (or Evolved Node Bs (ENBs)) cooperatively support downlink transmission for a terminal (or User Equipment (UE)).
2. Description of the Related Art
A variety of mobile communication standards, such as 3rd Generation Partnership Project (3GPP) High Speed Downlink Packet Access (HSDPA), 3GPP High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), Long Term Evolution-Advanced (LTE-A), 3GPP2 High Rate Packet Data (HRPD), and institute of Electrical and Electronics Engineers (IEEE) 802.16, have been developed to support high-speed, high-quality wireless packet data transmission services.
An LTE system maximizes a capacity of a wireless system using a variety of wireless access technologies. An LTE-A system, which evolved from the LTE system, improves data transmission capability compared to the LTE system.
The existing 3rd Generation (3G) wireless packet data communication systems, such as HSDPA, HSUPA and HRPD, use technologies such as Adaptive Modulation and Coding (AMC) and a channel-sensitive scheduling, which improve transmission efficiency. AMC and channel-sensitive scheduling may receive feedback, e.g., partial channel status information from a receiver, and apply an appropriate modulation and coding scheme at the time that is determined to be most efficient.
In a wireless packet data communication system to which AMC is applied, a transmitter may adjust an amount of its transmission data depending on the channel status. More specifically, if the channel status is poor, the transmitter reduces the amount of transmission data to set the receive error probability to a desired level. However, if the channel status is good, the transmitter increases the amount of transmission data to efficiently transmit more information while setting the receive error probability to the desired level.
In a wireless packet data communication system to which channel-sensitive scheduling is applied, a transmitter selectively serves a user having an excellent channel status among multiple users, so the system capacity increases, compared to allocating a channel to one user and serving the user. This capacity increase is often referred to as “multi-user diversity gain.” When used together with a Multiple Input Multiple Output (MIMO) transmission scheme, AMC may also include a function of determining a number of or ranks of spatial layers for transmission signals. In this case, the wireless packet data communication system, to which AMC is applied, also considers the number of layers on which it will transmit data using MIMO, without simply considering only the modulation scheme and coding rate, in determining the optimal data rate.
Generally, Orthogonal Frequency Division Multiple Access (OFDMA) contributes to an increase in capacity, compared to Code Division Multiple Access (CDMA). One of the several causes of contributing to an increase in the capacity is that OFDMA can perform frequency domain scheduling. Basically, capacity gain is obtained by the channel-sensitive scheduling method based on the time-varying characteristics of channels, and additional capacity gain can be obtained by utilizing the frequency-dependent characteristics of channels. Accordingly, many studies have been conducted to switch CDMA, i.e., a multiple access scheme that has been used in the 2G and 3G mobile communication systems, to OFDMA in the next-generation communication system.
FIG. 1 illustrates a conventional cellular mobile communication system in which a transmit/receive antenna is placed at a center of each cell.
Referring to FIG. 1, the cellular mobile communication system the cellular mobile communication system includes three cells, i.e., a cell#1 100, a cell #2 110, and a cell#3 120. A specific UE receives a mobile communication service that is provided based on the above-described several methods, from a selected cell for a semi-static period.
Herein, it is assumed that cell#1 100 provides a mobile communication service to a UE#1 101 and a UE#2 102 located in the coverage area (or service area) of cell#1 100, cell#2 110 provides a mobile communication service to a UE#3 111, and cell#3 120 provides a mobile communication service to a UE#4 121. Transmit/receive antennas 130, 131, and 132 are placed at the centers of the cell#1 100, the cell#2 110, and the cell#3 120, respectively. For example, the transmit/receive antennas 130, 131, and 132 may correspond to ENBs or relays.
The UE#2 102, which receives a mobile communication service via cell#1 100, is located farther away from the antenna 130, than the UE#1 101. Further, the UE#2 102 suffers significant interference from the antenna 132 in cell#3 120, which is adjacent to cell#1 100. Therefore, the UE#2 102 is lower than the UE#1 101 in terms of a data rate supported by cell#1 100.
If mobile communication services are independently provided in cell#1 100 to cell#3 120, a Reference Signal (RS), which is used by a UE to measure a downlink channel status for each cell, is transmitted to the UE. For example, in a 3GPP LTE-A system, a UE receives a Channel Status Information-Reference Signal (CSI-RS) transmitted by an ENB, measures a channel status between the ENB and itself, based on the received CSI-RS, and feeds back channel status in a feedback mode determined by the ENB, at a timing that also determined by the ENB.
FIG. 2 illustrates resource positions of CSI-RSs transmitted from an ENB to a UE in a conventional LTE-A system.
Referring to FIG. 2, resources available in the LTE-A system are divided into the same-sized resource blocks, where the horizontal and vertical axes of the resources correspond to a time axis and a frequency axis, respectively.
Signals for two CSI-RS antenna ports are transmitted on resources of resource blocks 200 to 219. An ENB transmits two CSI-RSs for downlink measurement to a UE on the resources of the resource block 200.
For the cellular mobile communication system having a plurality of cells as illustrated in FIG. 1, a resource block having its own unique position is allocated for each cell, and a CSI-RS is transmitted on resources of the allocated resource block. For example, cell#1 100 transmits a CSI-RS on resources of resource block 200 in FIG. 2, and cell#2 110 transmits a CSI-RS on resources of resource block 205 in FIG. 2. Further, cell#3 120 transmits a CSI-RS on resources of resource block 210 in FIG. 2. By allocating resource blocks (i.e., time and frequency resources) for CSI-RS transmission in different positions for individual cells, it is possible to prevent CSI-RSs from different cells from interfering with each other.
In the cellular mobile communication system, a UE located at an edge of a cell has a limited high data rate support capacity because it suffers significant interference from adjacent other cells. More specifically, high data rates provided to UEs located in a cell are significantly affected by the locations of the UEs within the cell. Basically, in a conventional cellular mobile communication system, a UE located relatively closer to the center of a cell may transmit and receive data at a higher data ratet than a UE located relatively farther away from the center of a cell.